1. Field of the Invention
This invention relates to a pneumatic tire improving drainage performance and steering stability and reducing noise without sacrificing other tire performances.
2. Description of Related Art
As shown in FIG. 5, a tread pattern of a pneumatic tire, e.g. passenger car tire is usual to be defined by circumferential grooves 101-103 extending in parallel to an equatorial plane of the tire and a plurality of slant grooves 104-106 opening to the circumferential grooves 101, 102 and extending obliquely with respect to the equatorial plane toward each end of a tread portion.
In such a tread pattern, drainage in front and back directions of the tire is mainly conducted by the circumferential grooves and drainage toward the side of the tire is conducted by the slant grooves, whereby an excellent drainage performance can be given to the tire.
In order to more improve the drainage performance in the tire having such a tread pattern, it is useful to widen a groove width to increase a groove area or so-called negative ratio, or to render the slant groove into such so-called high-angle arranging form that an inclination angle of a groove portion located in a central region of the tread with respect to the equatorial plane is made relatively small and an inclination angle of a groove portion located in each side region of the tread with respect to the equatorial plane is made relatively large.
However, when the negative ratio is increased only by widening the groove width or the like, there is a problem that the pattern noise is apt to be easily increased through the drainage performance is improved.
On the other hand, when the slant grooves are arranged in the high-angle form, there is a problem that the stiffness of the slant land portion defined by the slant grooves is apt to be lacking and the sufficient steering stability is not obtained.
Further, when the slant groove is opened to the circumferential groove, water flowing in the slant groove collides with water flowing in the circumferential groove at a position of an opening part to cause disorder of water flow, create bubbles and the like, so that there is a problem that the drainage performance is not obtained to an expected level.
From a viewpoint of noise reduction, it is effective to terminate the slant groove in the land portion without opening to the circumferential groove. In this case, however, there is generally a problem that it is difficult to obtain the sufficient drainage performance.
It is, therefore, an object of the invention to provide a pneumatic tire wherein all of drainage performance, steering stability and noise can be improved to satisfactory levels by adopting a tread pattern having a rib-shaped main land part arranged in a central region of a tread portion and rationalizing a plane shape (ground contact face) of a first slant land part defined by main slant grooves and arranging a first auxiliary land part of an adequate form between the rib-shaped main land part and the main slant groove.
For this end, the inventor has made various studies in order to develop tires having a tread pattern capable of realizing all of the improvements of drainage performance and steering stability and the reduction of noise, and found out that both of the steering stability and the drainage performance can effectively be improved by adopting a tread pattern useful for the reduction of noise, i.e. a tread pattern that a first auxiliary land part of an adequate form is arranged between a rib-shaped main land part and a main slant groove so as not to open the main slant groove to a first circumferential fine groove at least located in a central region, and rationalizing a plane shape (ground contact face) of a first slant land part defined by the main slant grooves.
According to the invention, there is the provision of a pneumatic tire comprising a tread portion divided into a central region and both side regions, at least one rib-shaped main land part formed in the central region and extending substantially in parallel to an equatorial plane of the tire, a plurality of main slant grooves extending obliquely from the central region toward each tread end of the tread portion with respect to the equatorial plane to form first slant land parts among these main slant grooves, in which the first slant land part has a plane form of approximately a triangle gradually increasing a width of the land part from a side of the equatorial plane toward a side of the tread end, and a first auxiliary land part having such a slope that a height of the land part gradually decreases toward a groove bottom of the main slant groove is arranged between the rib-shaped main land part and the main slant groove.
The term xe2x80x9ccentral regionxe2x80x9d used herein means a region centering on a position of the equatorial plane and corresponding to 5-50% of a tread width TW, and the term xe2x80x9cboth side regionsxe2x80x9d are regions located at both sides of the central region.
When the stiffness of the land part is important, the rib-shaped main land part is favorable to be integrally united with the first slant land part.
When it is required to more improve ground contacting property and drainage performance in the central region, it is favorable to arrange at least one first circumferential fine groove extending substantially in parallel to the equatorial plane in the rib-shaped main land part.
The first circumferential fine groove is favorable to have a sectional shape that a groove width becomes wide at a position of the groove bottom and becomes narrow at a position of a ground contact face of the tread portion. Particularly, the first circumferential fine groove is favorable to have a sectional shape of approximately a conical flask.
When the drainage performance is important, it is favorable that a plurality of slant sub-grooves each extending obliquely from the side region of the tread portion toward the tread end with respect to the equatorial plane are arranged to form second slant land parts among these slant sub-grooves in which a land width gradually increases toward the tread end.
Moreover, when the stiffness of the land part is important, it is favorable to integrally unite the first slant land part with the second slant land part.
When it is required to enhance the drainage performance toward the side of the tire, it is favorable that a second auxiliary land part having such a slope that a land height gradually decreases toward the groove bottom of the slant sub-groove is arranged between the first slant land part and the slant sub-groove.
When it is required to more enhance the ground contacting property and drainage performance, it is favorable that a second circumferential fine groove extending substantially in parallel to the equatorial plane and having a sectional shape similar to that of the first circumferential fine groove is arranged at a boundary between the first slant land part and the second slant land part.
When it is required to more balancedly improve the steering stability and the noise reduction, it is favorable that a first joint land part connecting the first slant land parts to each other and having an extremely narrow width is arranged along a groove wall of the second circumferential fine groove facing to the side of the equatorial plane and a third auxiliary land part having such a slope that a land height gradually decreases toward a groove bottom of the main slant groove is arranged between the first joint part and the main slant groove.
Moreover, it is more preferable that an angle between the ground contact face of the rib-shaped main land part and the slope of the first auxiliary land part, an angle between the ground contact face of the first slant land part and the slope of the second auxiliary land part and an angle between the ground contact face of the first joint part and the slope of the third auxiliary land part are within a range of 135-170xc2x0, respectively.